The present invention relates generally to an apparatus and method for retaining individual sheet substrates, especially paper or cardboard in a curved configuration during printing or while applying coating material, such as UV-curable coating.
It is often desirable to coat printed substrates, such as paper or cardboard, to thereby protect the printed surface from smudging and water damage. Particularly durable coatings of this type are those cured by exposure to UV rays. Typically, the substrate surface to be coated is dropped onto a conveyor where it moves toward a coating station having a fountain system, for example. The fountain system may include a continuous coating surface, such as a coating/transfer roller for applying a coating. If UV curable coating is to be used, the coated substrate is moved away from the coating station and toward a UV source where it is exposed to UV radiation for a predetermined amount of time to thereby cure the coating. Present methods and devices for coating substrates are typically designed for coating continuous webs of substrate dispensed from a roll, for example. Such processes and devices cannot easily accommodate individual sheets of substrate.
It is sometimes desirable to apply coatings to cut, individual sheets of substrate, rather than to a continuous web of material. A particular problem associated with the application of certain coatings, such as UV curable coatings, to individual sheets is the tendency of sheet substrates of lesser thicknesses to curl at the edges during coating. Further, it has been observed that individual sheets tend to adhere to the transfer roller during coating and do not release properly after coating. Similar problems have also been observed during printing, particularly printing of individual sheets having a relatively small thickness. Various means of correcting these problems have been utilized. For example, mechanical “fingers” may be used to hold the corners of the sheet substrate flat during coating. However, this solution has been found to be unacceptable, particularly in situations requiring the substrate to be completely coated, since the coating cannot reach the points of “finger” contact. Another unacceptable solution has been use of a flat vacuum conveyer. In this case, as an individual sheet substrate moves past the transfer roller, a vacuum through the conveyer holds the sheet flat and against the conveyer thereby preventing the sheet from adhering to the transfer roller. However, depending upon the thickness of the sheet substrate, undesirable “dimpling” may occur, caused by the sheet substrate being drawn into the apertures in the conveyor by the vacuum. Therefore, an improved method and apparatus for transferring coating from a transfer surface to individual sheets of substrate is desired.